BACKUP STGPOOL for dedupe runs about 6x slower than direct tape to tape.
Why?
1) First, the database has a huge number of random reads for dedupe rehydration.
Tack on any Dedup Deletion activity (SHOW DEDUPDELETEINFO) and anything else that’s competing for DB IOPS.
FIX: Put the database on SSD or RAM backed storage.
NOTE: SSD stats are usually lies. Sustained performance is 4500-12,000 IOPS each, divided by 2 for RAID-1/10, or by 3.5 for RAID-5/6)
FIX: increase server memory and provide more for DB2 bufferpools.
NOTE: This might require manually changing bufferpools, limiting filesystem cache, etc.
FIX: Large amounts of cache for the database containers
2) Next, the file class, while sequential, still has a large number of random read IOPS.
TSM Server has no read ahead for this. It reads the chunks in order, rather than requesting a huge buffer full of chunks.
As such, streaming speed will be limited by DB latency, file-class latency, and actual read IO times.
FIX: Reduce the latency for your file class
FIX: Reduce the latency for your database
FIX: Don’t do anything else during BACKUP STGPOOL.
FIX: Run your EXPIRE INVENTORY and IDENTIFY DUPLICATE after, not before.
FIX: Submit a Design Change Request (DCR) for larger chunk read cache to be used for BACKUP STGPOOL.
FIX: Submit a Design Change Request (DCR) for larger tape write buffer.
3) Last, tape buffer underruns can kill performance.
If the write buffer empties, then the tape will stop.
Before it begins again, the tape has to be repositioned backward.
For LTO drives, usually the minimum write speed is 50MB/sec.
Anything less, and you have latency and tape life consumed by “shoe shining”.
FIX: Fix/improve issues 1 and 2 above.
FIX: Submit a design change request to allow TSM to interleave more threads onto the same tape at once.
FIX: Use tape drives with lower minimum speeds to prevent underruns
FIX: Don’t use tape. Use virtual tape, another dedupe disk pool, or a replica target TSM server.
4) Check TSM server instrumentation.
This will show you where your time is spent, and what to upgrade next.
INSTRUMENTATION BEGIN
BACKUP STGPOOL DEDUP COPYPOOL
wait several minutes
INSTRUMENTATION END FILE=/tsm/instrumentation.out
